The present invention relates to a rolling element for use in a rolling bearing or a toroidal continuously variable transmission (CVT) for automobiles and a process for producing the rolling element. More specifically, this invention relates to a rolling element for the CVT which is improved in rolling-fatigue strength by preventing short-life peeling or flaking due to hydrogen-induced embrittlement that will be caused when hydrogen generated by decomposition of a lubricating oil during rolling of the rolling element penetrates into the metal material of the rolling element, and a process for producing the rolling element.
U.S. Pat. No. 5,556,348 discloses a toroidal CVT which includes input and output disks and a power roller disposed between the input and output disks. The input and output disks and the power roller are subjected to carburizing and grinding to improve fatigue fracture lives of traction surfaces of the input and output disks and power roller.
U.S. Pat. No. 6,051,080 discloses a power roller for a toroidal CVT which is improved in durability by carburizing and in rolling-fatigue strength of the bearing surface receiving balls by grinding. In addition, Japanese Patent Application First Publication No. 2000-291757 discloses a power roller for a toroidal CVT in which residual compression stress layers are formed on the bearing surfaces receiving balls by shot-peening. This technique contemplates to reduce the contact surface pressure produced when the balls roll on the bearing surfaces, to thereby restrain deterioration of fatigue life of the power roller.
However, these earlier techniques do not disclose positive suppression of the above-described short-life flaking due to the hydrogen-induced embrittlement. U.S. Pat. No. 5,510,974 discloses a grease-sealed bearing aiming at suppressing hydrogen infiltration into races of the bearing. The races have triiron tetroxide layers on the bearing surfaces which are formed by blackening treatment to thereby restrain the occurrence of flaking on the bearing surfaces and improve lives of the bearing.
Japanese Patent Application First Publication No. 6-313434 discloses a corrosion resistant rolling bearing in which a nickel plating layer is formed on a surface of at least one of an inner race, an outer race, rolling members and a retainer for the rolling members. This technique contemplates to improve corrosion resistance under the severe corrosive environment such as salt water spraying and enhance the plating ability.
In general, the toroidal CVT includes an input disk, an output disk and power rollers contacted with the input and output disks via a lubricating oil. Each of the disks has a traction surface contacted with a traction surface of an inner race of each power roller. Rotation of the input disk is transmitted to the output disk by the traction drive produced between the traction surfaces of the disks and power rollers. When the toroidal CVT is driven, a high loading force is applied to the traction surfaces of the input and output disks and the traction surfaces of the power rollers. This will cause a high contact surface pressure exerted on bearing surfaces of the inner and outer races of each power roller which are in rolling contact with rolling members such as steel balls. At this time, the maximum contact surface pressure may reach more than 3 GPa. Further, traction force and radial load are applied onto the bearing surfaces of the races of the power roller when the rolling members roll on the bearing surfaces. This may cause microscopic metal-to-metal contact between the bearing surfaces and the rolling members or increase rolling-friction resistance generated therebetween, whereby tangential force applied onto the bearing surfaces will become large so that rolling-fatigue lives of the races will be lowered.
In addition, it is known that a grease-lubricating bearing tends to be affected by the tribochemical reaction caused between the grease and the bearing surfaces of the races which are in contact with a plurality of rolling members. The tribochemical reaction will be promoted by a catalytic action of the neo-surface that is newly produced on the bearing surface by the microscopic metal-to-metal contact between the bearing surfaces and the rolling members. This will cause chemical decomposition of the grease, resulting in the production of hydrogen. The hydrogen produced will infiltrate into the metal structure of the races to thereby deteriorate the rolling-fatigue lives thereof.
In order to eliminate the above-described problem of the grease-lubricating bearing, there has been proposed the blackening treatment as disclosed in the above-described earlier technique. In the blackening treatment, the races are immersed in a caustic soda solution heated at a temperature of 130xc2x0 C.-160xc2x0 C. This will make adverse influence on working environment and therefore it is industrially undesirable. Further, the triiron tetroxide layers formed by the blackening treatment will not sufficiently remain on the bearing surfaces under the severe conditions such as high temperature and high contact surface pressure. Therefore, it will not be assured to suppress the hydrogen infiltration into the metal structure of the races.
There is a demand to solve the above-described problems in the earlier techniques. An object of the present invention is to provide a rolling element for a continuously variable transmission (CVT) which is free from the neo-surface production caused by the microscopic metal-to-metal contact and therefore suppresses the hydrogen infiltration into the metal structure of the rolling element, by forming a protection coat capable of preventing hydrogen from infiltrating therethrough into the metal structure of the rolling element. Specifically, the object of the present invention is to provide the rolling element that can be prevented from suffering from the short-life flaking due to the hydrogen-induced embrittlement which is caused by infiltration of the hydrogen generated by chemical decomposition of a lubricating oil upon rolling of the rolling element, into the metal structure of the rolling element, so as to be improved in rolling-fatigue life. It is another object of the present invention to provide a process for producing the rolling element using a relatively simple surface treatment. Still other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.
According to one aspect of the present invention, there is provided a rolling element for a continuously variable transmission, including input and output disks and a power roller interposed between the input and output disks, the power roller including an inner race, an outer race and a plurality of rolling members interposed between the inner and outer races, the input and output disks and the inner race having rolling contact surfaces coming into rolling contact with each other via lubricating oil, the inner and outer races having rolling contact surfaces coming into rolling contact with the rolling members via lubricating oil, the rolling element comprising:
a nickel-based coat formed on at least one of the rolling contact surfaces, the nickel-based coat having a thickness ranging from 0.1 to 20 xcexcm.
The nickel-based coat of the rolling element according to the invention can be formed by a relatively simple surface treatment. With the formation of the nickel-based coat, the rolling element can be free from the microscopic metal-to-metal contact and be sufficiently prevented from suffering from hydrogen infiltration into the metal structure of the rolling element. The rolling element can be significantly improved in the rolling-fatigue life, and the excellent property of the coat can be maintained for a long period. If the thickness of the coat is less than 0.1 xcexcm, the effects of reducing the microscopic metal-to-metal contact and suppressing the hydrogen infiltration into the metal structure of the rolling element will not be sufficiently exhibited. If the thickness of the coat is more than 20 xcexcm, the stress generated in the coat will become excessively large as the thickness of the coat increases, so that flaking of the coat will occur in a relatively early stage of use. This cannot sufficiently contribute to improvement in the anti-flaking property of the coat, namely, the rolling-fatigue life of the rolling element.
The thickness of the nickel-based coat is preferably in a range of 0.1 to 10 xcexcm. This can provide the stable quality of the rolling element which is required to perform the effects of reducing the microscopic metal-to-metal contact and suppressing the hydrogen infiltration into the metal structure of the rolling element. This can also improve productivity of the rolling element.
The thickness of the nickel-based coat is more preferably in a range of 0.5 to 7 xcexcm. This can provide the more stable quality of the rolling element required for the effects described above and can more improve productivity of the rolling element.
The surface roughness of the nickel-based coat may be not more than 0.1 in terms of arithmetical mean roughness Ra. This can suppress increase in metal-to-metal contact rate at the rolling contact portion of the rolling element and deterioration of the rolling-fatigue life of the rolling element which results from surface damage caused from an outer-most area of the rolling contact portion. If the surface roughness Ra of the coat is more than 0.1, the metal-to-metal contact rate at the rolling contact portion of the rolling element will increase, thereby causing softening of the metal material due to the temperature increase at the rolling contact portion, and surface damage at the rolling contact portion. This will result in deterioration of the rolling-fatigue life of the rolling element. Meanwhile, the measurement of arithmetical mean roughness Ra of the coat is carried out in accordance with JIS B 0601-1994 and JIS B 0651.
A base metal of the rolling element which is obtained after forming the nickel-based coat thereon may have a surface roughness of not more than 0.1 in terms of arithmetical mean roughness Ra at the rolling contact surface. This can suppress increase in metal-to-metal contact rate at the rolling contact portion of the rolling element and deterioration of the rolling-fatigue life of the rolling element which is caused by the surface damage starting from an outer-most area of the rolling contact portion, even if the almost part of the coat is dissipated by wear and the base metal is brought into direct rolling contact. If the surface roughness Ra of the base metal at the rolling contact surface may be more than 0.1, the metal-to-metal contact rate at the rolling contact portion of the rolling element will increase, so that the rolling-fatigue life of the rolling element will be deteriorated as explained above. The measurement of arithmetical mean roughness Ra of the base metal is carried out in accordance with JIS B 0601-1994 and JIS B 0651.
The nickel-based coat may have a Vickers hardness of not less than Hv 300. This can assure sufficient wear resistance of the coat and maintain the excellent property of the coat. If the hardness of the coat is less than Hv 300, the wear resistance of the coat will become insufficient so that the excellent property of the coat cannot be obtained. The measurement of the hardness Hv is carried out in accordance with JIS B 7725 and JIS Z 2244.
The nickel-based coat may have a Vickers hardness ranging from Hv 300 to Hv 700. This can maintain the wear resistance of the coat and reduce the stress that might be caused in the coat, even when the coat has a relatively large thickness, so that the coat can be prevented from suffering from crack or flaking due to embrittlement. If the hardness of the coat is less than Hv 300, the wear resistance of the coat will become insufficient as described above. If the hardness of the coat is more than Hv 700, the stress that will be caused in the coat tends to increase specially when the thickness of the coat is as large as 10 to 20 xcexcm. This will cause crack due to embrittlement of the coat under high contact surface pressure condition.
The nickel-based coat may contain phosphorus P in an amount of 0.1 to 12 mass percent. This can improve the wear resistance of the coat and assure the toughness thereof, so that the coat can be prevented from suffering from crack or flaking due to embrittlement and therefore the excellent property of the coat can be obtained. If the phosphorus P content is less than 0.1 mass percent, a sufficient wear resistance of the coat cannot be obtained. If the phosphorus P content is more than 12 mass percent, the toughness of the coat will decrease, thereby causing crack or flaking due to embrittlement of the coat.
According to a further aspect of the present invention, there is provided a continuously variable transmission, comprising:
input and output disks including a pair of first rolling contact surfaces opposed to each other, the input and output disks being arranged in a coaxial and spaced relation to each other;
a power roller interposed between the input and output disks, the power roller comprising:
a plurality of rolling members;
an inner race including a second rolling contact surface coming into rolling contact with the pair of first rolling contact surfaces via lubricating oil; and
an outer race opposed to the inner race,
the inner and outer races including a pair of third rolling contact surfaces coming into rolling contact with the plurality of rolling members via lubricating oil, and
a nickel-based coat formed on at least one selected from the pair of first rolling contact surfaces, the second rolling contact surface and the pair of third rolling contact surfaces, the nickel-based coat having a thickness ranging from 0.1 to 20 xcexcm.
The nickel-based coat may be formed on the bearing surfaces as the third rolling contact surfaces of the inner and outer races of the power roller. Since the nickel-based coat can withstand high contact surface pressure and high load applied to the bearing surfaces of the races, the races and the power roller can be improved in rolling-fatigue lives and the performance can be maintained for a long period of use. Further, the nickel-based coat may be formed on the traction surface as the second rolling contact surface of the inner race of the power roller and may be formed on the traction surfaces as the first rolling contact surfaces of the input and output disks. This can improve the rolling-fatigue lives of the power roller and the disks and the performance of the CVT as a whole can be increased.
According to a still further aspect of the present invention, there is provided a process for producing a rolling element for a continuously variable transmission, including input and output disks and a power roller interposed between the input and output disks, the power roller including an inner race, an outer race and a plurality of rolling members interposed between the inner and outer races, the input and output disks and the inner race having rolling contact surfaces coming into rolling contact with each other via lubricating oil, the inner and outer races having rolling contact surfaces coming into rolling contact with the rolling members via lubricating oil, the rolling element including a nickel-based coat formed on at least one of the rolling contact surfaces, the process comprising:
subjecting the at least one of the rolling contact surfaces to one of strike plating, electroplating, combination of strike plating and electroplating, and combination of strike plating and electroless plating to form the nickel-based coat thereon.
The process can provide a rolling element for a CVT which is prevented from suffering from microscopic metal-to-metal contact and hydrogen infiltration into the metal material of the rolling element to thereby be improved in the rolling-fatigue life, by using a relatively simple surface treatment. Namely, owing to forming the nickel-based coat on the rolling element, wear resistance of the coat and adhesion thereof relative to the metal material of the rolling element can be enhanced, so that the excellent property of the coat can be obtained.
The strike plating may be conducted at a current density of 0.1xc3x97102 to 10xc3x97102 A/m2 (0.1 to 10 A/dm2). This can increase the productivity and provide an adequate surface roughness of the coat to stabilize the quality of the rolling element. If the current density is less than 0.1xc3x97102 A/m2 (0.1 A/dm2), the productivity will be lowered to insufficient level. If the current density is more than 10xc3x97102 A/m2 (10 A/dm2), the productivity will be increased but the surface roughness of the coat will become large, making it difficult to assure the quality of the rolling element. Further, the strike plating is preferably conducted at a current density of 0.1xc3x97102 to 5xc3x97102 A/m2 (0.1 to 5 A/dm2). This can inhibit the decrease of the productivity to the full extent and provide an adequate surface roughness of the coat to stabilize the quality of the rolling element.
The electroplating may be conducted at a current density of 0.1xc3x97102 to 10xc3x97102 A/m2 (0.1 to 10 A/dm2). This can increase the productivity and provide an adequate surface roughness of the coat to stabilize the quality of the rolling element. If the current density is less than 0.1xc3x97102 A/m2 (0.1 A/dm2), the productivity will be lowered to insufficient level. If the current density is more than 10xc3x97102 A/m2 (10 A/dm2), the productivity will be increased but the surface roughness of the coat will become large, making it difficult to stabilize the quality of the rolling element.
The process may further include subjecting the at least one of the rolling contact surfaces to baking at a temperature of not more than 200xc2x0 C. after the plating treatment. Owing to the baking at the temperature of not more than 200xc2x0 C., softening of the metal material of the rolling element and reduction of the residual stress therein can be suppressed and sufficient effects of hydrogen removal can be provided. As a result, the more stabilized quality of the rolling element can be obtained. Namely, the hydrogen that is infiltrated into the coat or the metal material of the rolling element at the electroplating or electroless plating treatment and into the metal material thereof at the surface-hardening treatment such as carbonitriding, can be removed by the baking treatment. If the baking is conducted at a temperature of more than 200xc2x0 C., the amount of the hydrogen removed will increase, but the metal material will be softened by heat at the high temperature, or the residual stress as required in a portion of the metal material to which residual compression stress is applied by shot-peening or other suitable method, will be reduced. The baking treatment is preferably carried out within a vacuum furnace. This can enhance the effects of hydrogen removal.